1. Field of the Invention
The present invention relates to the field of wireless communication networks. Specifically, the present invention is directed at conserving battery power for a cellular telephone and reducing registration traffic in a cellular telephone system.
2. Description of Related Art
In the prior art GSM System, a service area, which can be a metropolitan city, is subdivided into location areas. Each location area contains one or more cells, each of which containing a base station which communicates with one or more mobile stations (i.e., one or more mobile telephones, each referred to as a "mobile"). The base stations, in turn, are connected to a base station controller. In addition, a mobile switching center (MSC) is used to communicate with all the base station controllers in the GSM system and to keep track of the latest position of each of the mobiles. Thus, the MSC knows in which location area each mobile is currently located.
In many parts of the GSM system, location areas are formed by grouping geographically proximate cells together. Thus, each location area is composed of either one cell or multiple adjacent cells. The mapping of cells to location areas is defined system-wide, so the assignments of cells to location areas are the same for all users. Each location area identification (LAI) table specifies the cell to location area mapping for a respective location area. The LAI table for each location area is contained in the MSC.
The GSM system needs to know the mobile's latest position because the MSC uses that information to "page" the mobile--i.e., the location information is used to notify the mobile of an incoming call. The MSC sends the paging message only to the cells contained in the location area in which the mobile is located so as to avoid the resource wasteful technique of sending a paging message over all the cells in the GSM system. For example, when the mobile moves from a first location area to a second location area in idle mode, the mobile signals the MSC to inform the GSM system that the mobile has changed location area. subsequently, when the GSM system wants to signal the mobile, the GSM system transmits the paging message only to the cells in the location area where the mobile is currently located.
FIG. 1 illustrates a prior art LAI table set 101 containing a mapping of cells to location areas. Each location area has an associated LAI table which contains an entry for each cell that is assigned to the location area. For example, cell X.sub.1, cell X.sub.2, cell X.sub.3 to cell X.sub.n are assigned to a first LAI table LAI X. Also, cell Y.sub.1, cell Y.sub.2, cell Y.sub.3 to cell Y.sub.n are assigned to a second LAI table LAI Y. Cell X.sub.1, cell X.sub.2, cell X.sub.3 to cell X.sub.n make up one continuous area while cell Y.sub.1, cell Y.sub.2, cell Y.sub.3 to cell Y.sub.n make up a second continuous area. If there is only one cell assigned to a location area, then the LAI table for that location area would contain only one entry.
As noted above, a mobile is required to notify the system when the mobile moves from one location area to another. Specifically, a mobile must inform the MSC when the mobile moves from a first location area to another location area. This notification is termed a "registration" and occurs only if the mobile is in idle mode and the mobile happens to change location areas. The traffic resulting from the mobile notifying the MSC of the location area change is referred to as "registration traffic."
When the mobile moves from cell X.sub.3, which is in location area LAI X to an adjacent cell Y.sub.3, which is in location LAI Y, the mobile is required to perform a registration with the MSC.
When a call comes through for the mobile, the MSC looks up the number of the mobile, and receives information as to the location area in which the mobile is currently located. The MSC will then send a paging message to be broadcasted in the one or more cells which are contained in the location area to notify the mobile of the incoming call.
Thus, when the mobile switches location areas, it will have to register with the GSM system to update the system on the mobile's current location. However, every time the mobile performs a registration, there are two expenses on resources: one is on the use of the capacity of the system itself (i.e., use of the bandwidth, or spectrum, resources of the GSM system). The other expense is on the power resource (i.e., the power of the battery of the mobile) consumed to transmit the registration traffic.
As the mapping of cells to location areas is fixed--i.e., all users have the same LAI mapping--excess registration traffic is generated by a user who generally moves between two adjacent cells that are contained in two separate location areas. Every time the user moves from the cell in the first location area to the cell in the second location area, registration traffic is generated by the mobile of the user. Similarly, registration traffic is also generated by the mobile every time the user moves back from the cell in the second location area to the cell in the first location area. Therefore, if the user has to travel between the border of two cells, each of which belonging to a different location area, there will be registration traffic even though the mobile will generally be located in this area.
FIG. 2 is a diagram of the prior art GSM system having a location area LA1 composed of a set of cells C1, C2, and C3, each having base station transceiver BST1, BST2, and BST3, respectively, located therein. Base station transceivers BST1, BST2, and BST3 communicate with base station controller BSC1 of a set of BSCs (BSC1 through BSCn) (not shown). Set of BSCs are connected to mobile switching center MSC1 (not shown). A mobile MS1, located in cell C3, communicates with MSC1 through the use of base station BST3 and base station controller BSC1.
Also contained in FIG. 2 is a location area LA2 composed of cells C4, C5, and C6, each having base station transceivers BST4, BST5, and BST6, respectively, located therein. Base station transceivers BST4, BST5, and BST6 communicate with base station controller BSC2 of set of BSCs (BSC1 through BSCn). As discussed above, set of BSCs are connected to mobile switching center MSC1.
As described above, to allow the GSM system to locate mobile MS1, mobile MS1 is responsible for notifying mobile switching center MSC1 every time mobile MS1 moves from one location area to another. For example, in FIG. 2, as mobile MS1 moves out of cell C3, which is a cell in the current location area, into cell C5, which is a cell in the surrounding location area, mobile MS1 will register with MSC1.
Also as noted above, registration traffic will be generated when a user who is generally located in one area made up of two adjacent cells, each from different location areas--e.g., cell C3 in location area LA1 and cell C5 in location area LA2--moves between the two cells constantly. This movement will generate registration traffic due to the fact that the movement profile of the user is not matched to the location area definition. Thus, it would be desirable to reduce this registration traffic on the network and allow the mobile to save power.